Medical implants for use as replacement structures in patients have become widespread in their application. In particular, orthopedic implants for replacing joints or other structures have received a great deal of attention commercially and scientifically. Oftentimes, orthopedic implants utilize a cement composition (e.g., a polymethyl methacrylate cement) to adhere the implant to bone. The bone/cement interface and cement/implant interface form two mechanical interlocking interfaces which are accountable for implant fixation.
The art generally recognizes that increasing the roughness of an implant surface, i.e., the arithmetic mean deviation of the surface profile, Ra, can result in a higher strength bond when cement is used to adhere the roughened surface to bone. Accordingly, enhanced cement/implant tensile strength bonding is often achieved by applying processes, such as grit blasting and/or glass bead shot blasting, to make progressively rougher implant surfaces In general, the relationship between increasing Ra and increasing bond strength is linear. Roughened implant surfaces, however, can be detrimental with regard to abrasion processes. If an implant's mechanical interlocking to the bone cement (e.g., a hardened polymethyl methacrylate (“PMMA”) composition) is not perfect, or degrades with time, relative movement (e.g., micromotion) between the bone cement and the implant can result in PMMA debris generation. While some believe a degree of micromotion is tolerable, PMMA debris has been found to be transported to the neighboring bone tissue, causing abnormal tissue response. Clinically, PMMA particles have been found to be directly associated with osteolysis around cemented hip stem implant. This threatens mid/long-term implant fixation. In general, rougher implant surfaces tend to result in higher debris generation (e.g., cement residues) and more trapped spaces in between PMMA cement and implant, which can decrease the ultimate fracture toughness of the bond. Thus, practitioners are faced with a tradeoff when deciding whether or not to increase the Ra of an implant surface to improve implant fixation.
Accordingly, a need persists for implants and methods of treating such implants such that cement bond strength can be maintained at a high level, preferably without resulting in enhanced degradation due to abrasion-related processes.